The invention relates to a method of applying a marking in a surface of an element comprising a substrate of aluminum or an alloy thereof with an anodized outer layer. The invention further relates to an element with an anodized surface of aluminum or an alloy thereof and having a region with a visual property which differs from the corresponding visual property of adjoining portions of the surface and which forms a visual marking which is observable to the human eye.
Protective layers of aluminum products obtained through anodizing in general have a high degree of hardness, a high resistance to wear, a good adhesion to the substrate, and a good heat resistance. This renders anodized surfaces highly suitable, for example, for use as protective layers on surfaces of practical objects which come into intensive contact with other surfaces such as electric irons, shavers, door knobs, pushbuttons, and the like. A problem here is that it is desirable in many cases to provide the surface with markings such as alphanumerical characters, logos, operating instructions, or decorative lines, patterns, or other structures. It is important in that case that the markings in at least a number of the above respects do not have substantially worse properties than the anodized layer, because the marking could then be damaged at an early stage by loads occurring during use against which the anodized surface is sufficiently resistant. For this reason, conventional techniques for providing markings, such as painting, silk-screen printing, and the provision of stickers, are unattractive in many cases. Many methods of marking anodized surfaces have accordingly been proposed which are more satisfactory as regards at least a number of the properties mentioned above.
A method of applying markings in an anodized aluminum layer is known from Japanese patent application with publication no. 07204871. The aluminum layer is first colored black in that a black pigment is introduced into the pores of the layer. Then the colored anodized layer is irradiated with a laser. The anodized layer is removed thereby where it is irradiated, so that a marking is obtained which contrasts in white against the background of the remaining anodized layer with black color. A disadvantage of this method is that the anodized layer is absent at the area of the markings, so that the aluminum is not protected there. This may lead, for example, to corrosion in the marking area, whereby the material properties as well as the sharpness of the marking are impaired.
Japanese patent application with publication no. 6-256993 describes a method in which this problem is counteracted in that the object is anodized once more. This, however, brings with it the disadvantage that an additional anodizing step is necessary.
French utility model 2 649 628 discloses an aluminum article which is first provided with an anodized layer, over which subsequently a layer is provided which is not resistant to laser radiation, whereupon markings are provided in the anodized layer through irradiation with a laser. This, however, has the disadvantage that the anodized layer is to be provided with an additional layer. This involves additional costs. Moreover, such a layer will usually not have as great a damage resistance, hardness, wear resistance, and heat resistance as an anodized layer.
A method and an element of the kind mentioned in the opening paragraph are known from German patent 195 09 497. This describes an anodized sole of an electric iron which is provided with markings in that the anodized layer is removed almost entirely along at least one line by means of a laser, such that a conversion layer comprising an aluminum oxide, aluminum, or an aluminum alloy with nitrogen included therein is formed by the anodized layer remaining behind in the processed surface. The conversion layer forms a protective layer against corrosion. Although the remaining conversion layer forms a protective layer and prevents portions of the anodized layer which adjoin the marking formed from breaking away, this method of marking of the anodized layer has the disadvantage that the markings form depressions of substantially the thickness of the anodized layer 25-70 (xcexcm), which is disadvantageous for the application of the layer as a sliding layer of, for example, an electric iron.
It is known from European patent application 0 121 150 to provide an anodized layer with markings by means of a laser beam in that substances are added to the anodized layer, such as glucose hydrate, cobalt acetate, zinc acetate, zinc nitrate, lead carbonate, chromium carbonate, ammonium vanadate, nickel nitrate, or nickel acetate, which undergo a contrasting discoloration when acted upon by a laser beam with a high energy density. A disadvantage of this method is that a special treatment is necessary for adding the substance with contrasting discoloration under the influence of laser radiation, and that the necessity of providing the substance with contrasting discoloration under the influence of laser radiation prevents the coloring of the anodized layer.
It is an object of the invention to provide a solution which renders it possible to obtain an anodized surface provided with a marking, whereby a smaller transition is present in the region where the marking adjoins neighboring portions of the anodized layer, whereby the application of special substances which change color under the influence of laser radiation is not necessary, and which can be readily combined with conventional methods of coloring anodized layers.
According to the invention, this object is achieved by a method of providing a marking in an anodized layer. The invention may furthermore be embodied in an element with an anodized surface of aluminum or an alloy thereof and having a region with a visual property which differs from the corresponding visual property of adjoining portions of the surface and which forms a visual marking which is observable to the human eye.
Since the laser beam penetrates the anodized outer layer at least for the major part and induces the change in the at least one visual property mainly locally in a zone of the region in which the anodized outer layer adjoins the substrate, zones of the anodized layer situated farther outward remain entirely or substantially unchanged, while the marking provided remains visible through the entirely or substantially unchanged anodized layer. The protecting effect of the anodized layer is not or substantially not detracted from thereby, and the layer remains substantially as free from unevennesses at its outer side as before the marking was applied. Furthermore, it is not necessary to introduce foreign substances into the anodized layer for providing the marking, so that the treatment required for this is dispensed with and no treatments are necessary which conflict with any coloring treatments.
Special embodiments of the invention have been defined in the dependent claims.